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How to correctly perform automotive power and ground tests

Monday, October 29, 2018 - 06:00
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I struggled with the concept of voltage drop testing until a contest held by my last employer embarrassed me enough to go home and do what was necessary to learn the technique. As a result, I found myself solving more electrical concerns for our customers and that didn’t go unnoticed by my boss. It didn’t take long for him to refer other techs in the shop to me when they were stumped by some electrical gremlin. While I certainly didn’t mind, I also didn’t want to do their work for them. After all, we were all on flat rate.

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And didn’t a wise man once say something about teaching a man to fish?

One instance that really stands out in my mind for some reason was a GMC blower motor that didn’t want to run at full speed. The tech working on the concern had ordered and installed a replacement blower motor only to have the new one act the same way. That’s when he came to me for help.

This is why many OEMs do not want you to backprobe. If you damage the weather seal, corrosion will not be far behind.

“Did you test power and ground to the blower motor?” I asked. He assured me he had but I was curious on exactly how he had performed the tests. So I asked him to show me.

Grabbing his test light, he disconnected the connector from the blower motor, turned the ignition key “on” and switched the blower motor control to “high.” He grounded his test light to a nearby screw head under the dash and placed the probe end of the test light in the connector socket bringing power to the motor. It lit up brightly. I didn’t really need to see him do anything more. I knew he had failed to test the power circuit correctly and I was willing to bet he hadn’t tested the ground side any more effectively.

Do you know why?

This particular job happened a long time ago, yet I still meet and hear stories from technicians who are making this same mistake every day. One foundational fact about testing for voltage drop is that the circuit MUST be “on” with current flowing. By disconnecting the connector from the blower motor, the tech was doing nothing more than measuring OCV (Open Circuit Voltage).

I went back to my bay and grabbed my PowerProbe, a tool I used frequently for electrical testing on most circuits. Another foundational rule is to always reference your meter to the battery so you can be assured of testing the entire circuit path. The other side of that rule is to get your measurement lead as close to the circuit load (in this case, the blower motor) as possible for the same reason. It is important to remember that you are only measuring the voltage potential (or lack thereof) between the two leads of your tool.

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